The use of atomic force microscopy to measure periodic electrical waveforms has been proposed previously in U.S. Pat. Nos. 5,381,101 and 5,488,305. The '101 and '305 patents (incorporated herein by reference) describe a system which can be used to make such measurements. This system is shown in FIG. 1 and comprises a system controller 10 connected to a conventional X-Y stage 12 and cantilever position detection optics 14. The controller 10 is typically implemented using a microprocessor or dedicated computer capable of supplying position control information to the X-Y stage 12. The stage 12 translates the IC sample under test (DUT) 16 to a desired position relative to a cantilevered tip 18 of a commercially available scanning probe microscope. The tip 18 is coupled to a cantilever 20 which can be displaced by the electric field between the tip 18 and the sample 16. The tip can be made of a conductive material such as aluminum or can be coated in a conductive material. Alternatively, the tip 18 can be formed integrally with the cantilever 20. Detection optics 14 provide a detection signal indicative of the displacement of the tip 18 relative to the surface of the sample 16. The detection optics 14 include a laser source 24 for illuminating the cantilever 20 with a focused optical beam. A portion of this beam is reflected by the cantilever and impinges on a conventional position detector 26. The resulting signal generated by the detector 26 can then be used to display the waveform.
In use, a repetitive test signal is applied to the DUT and appears as a periodic electrical waveform having a repetition frequency f at the surface of the DUT. A pulse generator 28 is used to apply a sampling signal to the cantilever and tip. The sampling signal is chosen to have a frequency which is offset from the repetition frequency f by a difference frequency .DELTA.f, the difference frequency being chosen to be less than the mechanical response frequency of the cantilever, i.e. the sampling signal has a frequency of f+.DELTA.f. The voltage across the gap between the tip and the DUT due to the periodic sample waveform and the sampling signal causes deflection of the cantilever which acts as a mechanical mixer in this mode and allows the waveform to be sampled and displayed by detecting the deflection of the cantilever.
FIG. 2 shows the signals involved in operation of this prior art system plotted in the time domain. The periodic waveform Vs has a frequency f (period 1/f). The sampling signal Vp has a frequency of f+.DELTA.f (period 1/(f+.DELTA.f)). Consequently, the sampling signal Vp progressively samples through the periodic waveform Vs, the tip deflection at each sampling point being indicative of the voltage in the periodic waveform at that point. This deflection is displayed directly on an oscilloscope as a time expanded waveform TE.
While the prior art approach is relatively easy to implement and the cantilever displacement directly provides the time expanded waveform which can be displayed on an oscilloscope, it is relatively inflexible when signals of arbitrary frequency are to be sampled and has poor noise performance since it is unable to average signals on a per point basis. Also, the use of a mixing frequency means that it is only possible to sample a complete waveform. Where an IC test pattern is very long, it is often desirable to sample only a part of the waveform in which a feature of interest is known to occur.
A variation on this prior art process is described in the PhD dissertation "Applications of Scanning Force Microscopy for Voltage Measurements with High Spatial and Temporal Resolutions" by Francis Ho, Stanford University, 1995. In this case, a portion of the waveform only is sampled. The sampling signal is not applied until the beginning of the portion of interest and is then swept through the portion on successive repetitions in the same manner as before. In one form, a single sweep is performed, in another, several sweeps are performed and the results integrated to improve signal to noise ratio. Other prior art techniques available to AFM measurements are sampling using Golay codes and sampling in a vacuum.
It is an object of the present invention to provide a method and apparatus which allows improved waveform sampling using atomic force microscopy.